Fischer Tropsch synthesis is a catalyzed process for converting synthesis gas (syngas) containing hydrogen and carbon monoxide into longer chain hydrocarbon products. Since its invention by Franz Fischer and Hans Tropsch during the 1920s, this process and variations on it have been commonly used in coal to liquids and gas to liquids operations. The Fischer-Tropsch condensation process is a reaction which converts carbon monoxide and hydrogen into longer chain, mostly paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g., 100 to 350° C., 125 to 300° C., or preferably 175 to 250° C.) and/or elevated pressures (e.g., 5 to 100 bar, or preferably 12 to 50 bar). The Fischer Tropsch reaction is exothermic, yielding roughly 145 kJ per “CH2” formed. The Fischer-Tropsch process can be used to prepare a range of hydrocarbon fuels, including LPG, naphtha, kerosene and gas oil fractions. Fischer Tropsch product selectivity is temperature and pressure dependent, so it is important for the heat of reaction to be quickly removed in order to achieve the desirable product mix and avoid catalyst damage. Increased temperatures may result in undesirable methane selectivity or carbon deposition. Steynberg, Andre, and Mark Dry. Fischer-Tropsch Technology. Amsterdam:4: Elsevier, 2006. Boiling water is commonly used as a coolant in Fischer Tropsch reactors, and the steam formed therefrom can be used to generate shaft work.
The concept of an organic Rankine cycle dates back at least as far as 1962, when H. Tabor and L. Bronicki patented vapor turbines with the object of providing “a method for the operation of turbines based upon the Rankine cycle in which the vapors leaving the exhaust are normally superheated”. See U.S. Pat. No. 3,040,528 to Tabor et al. By implementing a recuperator system, Tabor and Bronicki were able to improve the efficiency of a Rankine cycle based on the use of heavy fluids to be comparable to or better than the efficiency of a similar system based on the use of steam. Organic Rankine cycles have evolved over the last fifty years and are now widely used methods for generating electricity from low temperature heat sources. See Quoilin, Sylvain, and Vincent Lemort. “Technological and Economical Survey of Organic Rankine Cycle Systems.” 5th European Conference on Economics and Management of Energy in Industry (14-17 Apr 2009).
Many working fluids for these organic Rankine cycles have been proposed and developed. Recent patents include the development of a working fluid that is a “commercially available organic, which is operable under a broad range of temperatures, is thermally stable, has a high auto-ignition temperature, low freezing point and high critical temperature and is benign to the environment, and safe for human use.” See U.S. Pat. No. 7,225,621 to Zimron et al., which is herein incorporated by reference. The Fischer Tropsch process has been suggested as a potential heat source for one variation of an organic Rankine cycle called a Dual-Source Organic Rankine Cycle (DORC). The inventor of the DORC has stated that “there are a number of very important (and neglected) cases where substantial amounts of low-grade and mid-grade waste heat may be available simultaneously. Fischer-Tropsch Synthesis (FTS) reactors reject hundreds of megawatts at 500 K to 650 K, and lesser amounts are rejected in condensers at lower temperatures.” See U.S. Patent Appl. Pub. No. 20100300093A1 (Ser. No. 12/673,554.) filed 25 Nov. 2007 to Doty, F. D. entitled “High-Temperature Dual-Source Organic Rankine Cycle with Gas Separations,” which is herein incorporated by reference. Accordingly, there is an opportunity to develop improved processes for recovering the waste heat from a Fischer Tropsch process using an organic Rankine cycle.